Interaction of arsenic(III) and arsenic(V) on manganese dioxide: XPS and electrochemical investigations.

Manganese dioxide (MnO2) synthesized by solid-state reaction was characterized and sorption of As(III) and As(V) on it was studied in batch mode using 76As radiotracer. Arsenic removal efficiency was ∼98 % in the pH range of 3-9. Solvent extraction study indicated that >95% of arsenic is present as As(V) after sorption. A new electrochemical method was developed for explaining the arsenic-manganese interactions. Cyclic voltammetry and chronopotentiometry measurements were carried out which indicated the difference in the interaction of As(III) and As(V) with MnO2. X-ray Photoelectron Spectroscopy (XPS) was carried out in which the 3p3/2 binding energy peak of As(III) and As(V) standards was compared with the binding energy peaks observed for arsenic sorbed on manganese dioxide. The binding energy peaks of arsenic on MnO2 were matching with that of As(V), irrespective of the oxidation state of arsenic taken for sorption. The study confirmed that irrespective of the initial oxidation state, arsenic was sorbed on MnO2 as As(V); during the oxidation of As(III) by MnO2, manganese was reduced to Mn(II) and the Mn(II) formed during sorption was sorbed on the surface creating fresh surface promoting further sorption. Based on the observations, a mechanism of sorption has been proposed.